1. Field of the Invention
This invention relates generally to a speed control device for d-c motors, and more particularly to a speed control device for d-c motors which is capable of controlling motor operation at any desired speed settings.
2. Description of the Prior Art
A conventional speed control device for controlling the speed of a d-c motor typically includes a circuit arrangement as shown in FIG. 1. This arrangement includes a reactor or inductor L.sub.1 ; a capacitor C.sub.1, a diode D.sub.1 ; resistors R.sub.1, through R.sub.3 ; a variable resistor VR.sub.1 ; a d-c motor M; and a control circuit 1, which may be a commercially available IC such as NEC IC for motor controlling .mu.PC 147OH.
According to the prior art arrangement as shown in FIG. 1, a resistance circuit comprising the resistors R.sub.1 through R.sub.3 and the variable resistor VR.sub.1 is connected across the terminals of the d-c motor M. In the control circuit 1, while a current flowing in the d-c motor M is directed from the terminal D to the terminal B, the current is controlled so as to keep at a constant level a counter-electromotive force generated in the d-c motor M. In the example shown in the figure, the current flowing in the terminal D is adapted to be proportional to the current entering into the terminal C, and the terminal A is used for starting the d-c motor M. In this case, the speed (revolution) of the d-c motor can be controlled by changing the resistance value of the variable resistor VR.sub.1 . And, the changes in motor revolution caused by the temperature characteristics of the d-c motor are compensated by the changes with temperature in the terminal voltage V.sub.F of the diode D.sub.1. In the conventional type of control circuit shown in FIG. 1, however, it is necessary to additionally provide a diode D.sub.1, making the circuit configuration complex. Furthermore, in the conventional control circuit where motor revolution is set by setting the resistance value r of the variable resistor VR.sub.1, a voltage applied to the motor M is proportional to 1/r. As a result, the relationship between the amount of change in the resistance r and the revolution N of the motor tends to be nonlinear, making it difficult to ensure accurate control of motor speed.